Professor Dr. RANDOLF POHL

In May 2016 Randolf Pohl was appointed professor for atomic physics at the Cluster of Excellence PRISMA, Department of Physics, Mathematics and Computer Sciences of the Johannes Gutenberg University. His research focuses on laser spectroscopy of "exotic" muonic atoms and precision spectroscopy of atomic hydrogen.

In muonic atoms a negatively charged muon is bound to the atomic nucleus. With a mass 200 times as high, muons are the "heavier siblings" of electrons. Hence the Bohr radius of muonic atoms is 200 times smaller than that of "ordinary" atoms and the muon can "see" the atomic nucleus and its structure particularly well. This has an effect on the energy levels of muonic atoms which are measured using laser spectroscopy. The experiments are carried out at the worlds strongest muon beam at the Paul-Scherrer-Institute in Switzerland where we have been measuring the proton and other light nuclei since 2009 ... with consistently surprising results. We have for example „shrunk the proton“ by 4%! Ongoing and future projects at the JGU include the measurement of both the magnetic radius of the proton and of helium-3, the charge radii of lithium, beryllium and boron by use of laser spectroscopy and of heavy nuclei like radium by use of X-ray spectroscopy of muonic atoms.

Precision spectroscopy is applied to test quantum electrodynamics of bound states, to search for "new physics" beyond the Standard Model, and to determine fundamental physical constants like the Rydberg constant. And an astounding precision is achieved: In the research group of Nobel Prize winner Theodor W. Hänsch at MPI for Quantum Optics Munich we have determined the 1S-2S-transition in atomic hydrogen and deuterium with a precision of a few parts in 1015. The newly established workgroup at JGU is planning a hydrogen atomic clock and, for the first time, the precise laser spectroscopy of atomic tritium to dermine the charge radius of triton.

Randolf Pohl was born in Munich in 1970. He studied phyiscs at the Technical University Munich where he got his Diploma in laser spectroscopy of antiproton helium atoms at CERN. He did his PhD at ETH Zurich on the first observation of long-lived muonic atoms in the metastable 2S-state. The laser spectroscopy of this 2S-state in muonic hydrogen, deuterium and helium nuclei was in the centre of his research focus.In 2009, a 32-strong international collaboration under his leadership succeeded in measuring the proton in muonic hydrogen for the first time. The value is 10 times more precise yet 4% or 5 standard deviations smaller than the accepted average value of hydrogen and electron-proton scattering at that time. This "proton radius puzzle" is one of the biggest discrepancies in the Standard Model. In 2016 it was shown in the muonic deuterium that also the deuteron is significantly smaller than the hitherto accepted value.